The invention relates broadly to a method for removing heat from a chemical reaction. In particular, the invention is directed to a means for removing heat from an exothermic reaction of a liquid medium.
A chemical reaction in which heat is generated is said to be an exothermic reaction. An exothermic reaction is involved in the manufacture of some polymeric materials, for example, styrene-butadiene latex. Because of this exothermic reaction there are difinite problems inherent in the latex manufacturing process.
One reason for the problem is that the butadiene monomer employed in the basic reaction has a high vapor pressure. When these materials are brought together in a reactive vessel, therefore, the rate of feed into the reactor must be fairly slow. If the feed is too rapid, the heat produced in the exothermic reaction will cause the gases in the reactor to expand very rapidly and will create a dangerous overpressure condition. Another problem is that the latex product will suffer degradation if the temperature in the reactor vessel becomes too high.
In the latex process it is not commerically feasible to carry out the reaction by feeding monomeric materials into the reactor at a slow rate. An alternative, therefore, is to find a practical way to remove heat from the reaction. One technique which has been used for heat removal involves adding a reflux condenser to the reactor vessel. The condenser used in this technique is basically a shell and tube heat exchanger. This condenser, which is referred to as the knockback design, is connected into an outlet at the top of the reactor by a short length of conduit.
A common technique for making the styrene-butadiene latex involves a process referred to as an emulsion polymerization reaction. For this reaction the starting material which is charged to the reactor vessel is a suspension of latex seed particles in a water medium. Over a period of from about two and a half hours to seven hours an aqueous mix and a monomer mix are added to the latex solution, with continuous agitation of the mix. Ingredients of the aqueous mix include water, an initiator catalyst, a buffer, and a surfactant. The basic composition of the monomer mix is styrene, butadiene and a chain transfer agent.
To avoid the overpressure condition, and the possibility of product degradation, as mentioned above, the objective is to keep the temperature in the reactor at a specific set point, usually between about 80.degree. C. to 100.degree. C. This is achieved by condensing butadiene, which is vaporized by the heat of reaction. To explain further, since the butadiene will boil at the temperature and pressure conditions in the reactor, the vapors can be condensed by passing them directly from the reactor into the condenser. When the butadiene vapors condense in the tube bundle in the heat exchanger, the resulting liquid flows directly back into the reactor vessel to cool the reaction mass.
The knockback condenser, however, has certain disadvantages which make it unsuitable for use in the latex process. For example, several materials enter the reactor, along with the monomer mix, which are not condensible at the reaction temperatures and pressures. These noncondensibles include gases such as methane, butenes and nitrogen, which are carried into the condenser along with the butadiene vapor. It is necessary, therefore, to periodically remove the noncondensible gases from the condenser. If this is not done the gases will accummulate in the tube bundle and seriously reduce the heat transfer capability of the condenser.
The noncondensible gases are removed from the condenser by frequent venting through a header section located above the tube bundle. However, the act of venting the noncondensible gases, and the condensing of the butadiene vapors, causes the pressure in the tube bundle of the condenser to drop considerably below the pressure in the reactor vessel. This pressure differential provides a substantial driving force which permits some of the latex emulsion to surge upwardly from the reactor through the short entry conduit and into the tube bundle of the condenser. Since the latex emulsion is an extremely sticky material, it will readily adhere to the walls of the tubes of the condenser. In a short time the latex buildup on the tubes will seriously impair the heat transfer capability of the condenser. As a result, the condenser must be frequently cleaned to prevent it from completely plugging with the latex material.